1. Field of the Invention
The present invention relates to a method of radical polymerization of a vinyl monomer in the presence of a specific substance, and particularly, to a process for efficiently producing a vinyl polymer which has a high molecular weight desirable for practical properties such as mechanical properties, heat resistance and the like, and which has controlled molecular weight distribution.
2. Description of the Prior Art
In conventional radical polymerization, the molecular weight of the produced polymer strongly depends on the polymerization temperature, that is, the molecular weight of the polymer obtained is restricted by the polymerization temperature. Although the polymerization temperature is typically lowered in order to achieve a high molecular weight, decrease in polymerization temperature will elongate the polymerization time and increase the amount of remaining monomer. It becomes, therefore, difficult to effect an efficient production. Similarly, although the polymerization temperature is typically elevated in order to achieve a low molecular weight, increase in polymerization temperature will so accelerate the polymerization rate that is becomes difficult to control the reaction. For these reasons, there is a wide need for a polymerization method of freely controlling the molecular weight in a radical polymerization.
As an attempt to control the molecular weight in a radical polymerization, Tatemoto et al. have reported that radical polymerization of a fluorine-containing monomer such as tetrafluoroethylene proceeds in a manner like living polymerization in the presence of an iodine compound such as CF.sub.3).sub.2 CF-I (Shozo Tatermoto, Koubunshi-Ronbun-Shu, 49, 765 (1992)). Likewise, Otsu et al. have found that, when certain sulfur compounds were used, radical polymerization of styrene or the like occurred under light irradiation and that the extent of conversion and the molecular weight of the produced polymer increased with the time (J. Polym. sci.; part A; polym. chem., 32, 2911 (1994)). According to these methods, it is possible to control the molecular weight in a considerably wide range. However, these methods lack universality because they require a special polymerization initiator or a use of limited kinds of monomer.
In anionic polymerization, a precise control of the molecular weight is possible by a polymerization method called living polymerization. Recently, it has also become popular to research a polymerization system which enables living polymerization in radical polymerization. For example, it has been shown in Japanese Patent Publication No. 94-199916 A that, by using a radical polymerization initiator such as benzoly peroxide together with a stable free radical agent such as 2,2,6,6-tetramethyl-piperidinyloxy (TEMPO), a living-like polymerization of styrene can be achieved. According to this method, by selecting a polymerization condition, the molecular weight of the polymer obtained will depend not only on the polymerization temperature but also on the amounts of the initiator and the stable free radical agent used, so that one can control the molecular weight by those amounts to a certain extent.
Thus, when the above method is used, one can obtain a resin having a controlled molecular weight. However, this method has a drawback that the reaction rate in this method is so slow that it takes considerably more time compared with the usual radical polymerization to reach a high extent of conversion. In addition, coloration of the obtained polymer may sometimes occur depending on the reaction condition. Furthermore, TEMPO requires special attention in its handling and storage because it exists in radical state, and it is also very expensive. This method has thus some shortcomings that, for example, it requires more production costs compared with the conventional methods.
In view of such a situation, the present inventors have concentrated their efforts on controlling the radical reaction of vinyl monomer with the aim of increasing productivity and improving practical physical properties of radically polymerized vinyl polymers. In result, we have found by chance a method for obtaining a vinyl polymer having a high molecular weight and a restrained molecular weight distribution while retaining a high polymerization rate by including a specific substituted alkylhydroxylamine in the polymerization system, and thus completed the present invention.